The broad purpose of this study is to investigate certain catecholamine-rich cells considered responsible for modifying vascular smooth muscle contraction. These cells - small intensely fluorescent (SIF) cells of sympathetic ganglia and catecholamine containing cell bodies and terminals in the A2 region of brain stem - are presumed necessary for normal adjustments of vascular tone. Although this aspect of cardiovascular innervation is of especial theoretical and potentially practical importance, it has received scant attention. 1. Cardiovascular responses (blood flow rate, heart rate, blood pressure) will be monitored after treatments (electrical stimulation, neurotoxin, transmission blockade or depleting agent) designed to modify activity of brainstem catecholamine cells and SIF cells of rat superior sympathetic ganglion. 2. Also, influence of paracervical SIF cells on the uterine arteries will be studied using the estrogen-induced uterine hyperemia mode. 3. Understanding of the functional significance of SIF cell clustering beside ganglionic blood vessels will be sought using fluorescent dye experiments. 4. Changes in arterial blood flow, pressure, etc., will also be studied on stimulation and after neurotoxin administration to A2. Subsequent to the various treatments designed to influence activity of catecholamine-rich cells, fluorescence microscopy, electron microscopy, and electrophysiological recording will be used to obtain evidence that will be correlated with the cardiovascular responses observed. 5. As an extension of this work, the SIF cells and catecholamine rich elements of A2 will be studied in 2 models of hypertension: the spontaneously hypertensive rat (SHR) and the DOCA hypertensive rat.